The advent of plastics has introduced improved methods of packaging goods. For example, polyethylene and polypropylene plastic films, bags, bottles and styrofoam cups and blister packages have the advantages of being chemically resistant, relatively unbreakable, light in weight and translucent or transparent. The increasing use of plastics in packaging applications has created a serious waste disposal problem. Burning of these plastic materials is unsatisfactory since its adds to air pollution problems. Furthermore, many biodegradable-type materials are packaged in such plastic trash bags which pose an impediment to the natural biodegradation of the contents of the plastic trash bags.
Unlike some other packaging materials, such as paper and cardboard, plastics are not readily destroyed by elements of nature. Thus, burying them is not an effective means of disposal, and can be expensive.
Plastics are a biologically recent development, and hence, are not easily degradable by micro-organisms which degrade most other forms of organic matter and return them to the biological life cycle. As such, plastic containers and packaging films are beginning to litter the countryside after being discarded by careless individuals and are accumulating as a result thereof. Furthermore, the return of degradable materials to the biological life cycle is being hindered by disposing these trash materials in plastic trash bags.
Several approaches to the enhancement of the environmental degradability of plastics have been suggested. These include: (1) the incorporation of particulate biodegradable materials such as starch as "fillers"; (2) the introduction of photoderadation-sensitizing groups into the molecular structure of a polymer by copolymerization of a common monomer with a second monomer possessing such groups; and (3) the incorporation of small amounts of selected additives which accelerate oxidative and/or photo-oxidative degradation. The last approach is particularly attractive for the following reasons. First, the physical properties of the additive-containing compositions are extremely similar to those of the basic polymer. Second, existing compounding and fabrication processes and equipment can be utilized in the manufacture of finished products; hence, the cost of the finished product should be relatively low. Third, the sensitivity of the composition to environmental degradation can be controlled by a proper selection of the type and concentration of additives.
The enhancement of the rate of environmental deterioration of plastics through the use of certain oxidation-promoting additives is known in the prior art. For example, the preparation of degradable polyolefin films containing certain organic derivatives of transition metals is described in U.S. Pat. No. 3,454,510.
While transition metal compounds are effective accelerators of oxidative and photo-oxidative degradation processes, the use of such materials in the preparation of useful plastic compositions having enhanced degradability suffers from several limitations. The high reactivity of plastic compositions containing transition metal compounds leads to rapid degradation during high temperature extrusion and molding processes, with the result that such processes are difficult to control and tend to produce materials having poor mechanical and chemical properties. Some transition metal compounds impart undesired coloration to plastic compositions into which they are incorporated. Furthermore, the toxicity of some transition metal compounds limits their utility as additives in compositions intended for food packaging.